Immunophilins are a family of soluble proteins that mediate the actions of immunosuppressant drugs such as cyclosporin A, FK506 and rapamycin. Of particular interest is the 12 kDa immunophilin, FK-506 binding protein (FKBP12). FKBP12 binds FK-506 and rapamycin, leading to an inhibition of T-cell activation and proliferation. Interestingly, the mechanism of action of FK-506 and rapamycin are different. For review, see, S. H. Solomon et al., Nature Med., 1, pp. 32-37 (1995).
FK-506 binds to FKBP12 and the resulting complex binds to and inhibits calcineurin, a cytoplasmic phosphatase. The phosphatase activity of calcineurin is necessary for dephosphorylation and subsequent translocation into the nucleus of the transcription factor NF-AT. NF-AT causes interleukin-2 gene activation which in turn mediates T-cell proliferation.
The rapamycin-FKBP12 complex, on the other hand, associates with a protein of unknown function, termed RAFT1/FRAP. This tripartite complex is known to inhibit various kinases in the cell (i.e., p70S6, p34cdc2, cdk2) which are necessary for cell cycle progression in T-cells. Rapamycin is also known to be a potent antagonist of FK-506, presumably by acting as a competitive inhibitor for the FKBP12 binding.
More recently, it has been discovered that FKBP plays other important roles in the body. It has been found that FKBP12 forms a complex with the intracellular calcium ion channels--the ryanodine receptor (RyR) and the inositol 1,4,5-triphosphate receptor (IP.sub.3 R) [T. Jayaraman et al., J. Biol. Chem., 267, pp. 9474-77 (1992); A. M. Cameron et al., Proc. Natl. Acad. Sci. USA, 92, pp. 1784-44 (1995)], helping to stabilize calcium release. The ryanodine receptor has been found in skeletal muscle, cardiac muscle, brain and other excitable tissues. IP.sub.3 R mediates intracellular calcium release elicited by hormones and neurotransmitters that act at the cell surface to activate phospholipase C and generate inositol 1,4,5-triphophase (IP.sub.3). Most IP.sub.3 R is found associated with the endoplasmic reticulum, but some may occur on the cell surface and mediate calcium flux into the cell.
For both the RyR and the IP.sub.3 R, it has been demonstrated that FK506 and rapamycin are capable of dissociating FKBP12 from the receptor. In each case, the "stripping" off of FKBP12 leads to increased leakiness of the calcium channel and lower intracellular calcium concentrations.
Another role of FKBP12 is the regulation of neurite outgrowth in nerve cells. W. E. Lyons et al. [Proc. Natl. Acad. Sci. USA, 91, pp. 3191-95 (1994)] demonstrated that FK506 acts synergistically with nerve growth factor (NGF) in stimulating neurite outgrowth in a rat pheochromocytoma cell line. Interestingly, rapamycin did not inhibit the effects of FK-506 on neurite outgrowth, but rather was neurotrophic itself, displaying an additive effect with FK-506. In sensory ganglia, FK-506 demonstrated similar neurotrophic effects, but those effects were blocked by rapamycin. These results led the authors to speculate that FK-506 was exerting its neurotrophic effect through its complexing with FKBP12 and calcineurin and inhibition of the latter's phosphatase activity. Alternatively, the authors proposed FK-506 was acting via a "stripping" mechanism, such as that involved in the removal of FKBP12 from RyR and IP.sub.3 R.
In view of the wide variety of disorders that may be treated by stimulating neurite outgrowth and the relatively few FKBP12-binding compounds that are known to possess this property, there remains a great need for additional neurotrophic, FKBP12-binding compounds.